True flies are insects of the Order Diptera (Greek: di = two, and pteron = wing), possessing a single pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax. The common housefly is a true fly and is one of the most widely distributed animals.

The presence of a single pair of wings distinguishes true flies from other insects with "fly" in their name, such as mayflies, dragonflies, damselflies, stoneflies, whitefly, fireflies, alderflies, dobsonflies, snakeflies, sawflies, caddisflies, butterflies or scorpionflies. Some true flies have become secondarily wingless, especially in the superfamily Hippoboscoidea, or among those that are inquilines in social insect colonies.

It is a large order, containing an estimated 240,000 species of mosquitos, gnats, midges and others, although under half of these (about 120,000 species) have been described [1]. It is one of the major insect orders both in terms of ecological and human (medical and economic) importance. The Diptera, in particular the mosquitoes (Culicidae), are of great importance as disease transmitters, acting as vectors for malaria, dengue, West Nile virus, yellow fever and other infectious diseases.

Flies are prevalent in virtually all habitats, with over 16,000 species in North America. Flies can be distinguished from all other insects in that they only have one pair of normal wings. The other pair has evolved into small ball-like structures called halteres, thought to be used as stabilizing organs during flight. Most flies have compound eyes and mouthparts adapted for piercing, lapping or sucking fluids. Flies exhibit complete metamorphosis. Flies are some of the most deadly carriers of disease; millions still die from mosquito-carried malaria every year; countless thousands fall victim to yellow fever, typhoid and dysentery, all caused by fly-born bacteria. On the other hand, flies are important pollinators, scavengers, and parasites of other harmful insects.